Solid solution strengthening is one of the most important methods to enhance the strength of materials by alloying other elements into pure metals but still remain entirely as a solution. The strengthening effect is achieved by interacting a solute atom with dislocations either through an atomic size misfit or a modulus misfit. Unlike precipitation or strain (work) strengthened materials, which lose their strength in high temperature due to precipitation growth and strain relaxation in recovering and recrystallization at high temperature, solid solution alloying thus is practically useful in designing structural materials specially for high temperature use. Most commercially available solid solution alloys contain 2 to 3 elements, e.g. Ta-10W and Ta-8W-2Hf, with one element being the major component. Recently, a new type of structural materials has been identified as multi-component high-entropy alloys (HEAs) which normally comprise more than four metal elements with approximately equiatomic ratio, resulting in high entropy of mixing. When all elements in an alloy have an equal atomic ratio, the configuration entropy (also called mixing entropy), ΔSmix, will reach its maximum value ΔSmix=RInN (N is the number of elements and R the gas constant). Such alloys may have unique physical and mechanical properties because they still have simple crystal structure but their lattices were highly distorted due to atomic size misfit. The dislocation structures that govern material plasticity may be completely different from the traditional alloys.